Using pore-network modeling and characterization, we have shown that it is possible to tune the porous geopolymer micro- structure and the resulting molecular diffusion coefficients of drugs incorporated in this structure to span over two orders in magnitude [10]. It has also been shown that Fentanyl was released at a considerably higher rate under low pH conditions (pH 1; mimicking acidic stomach conditions) as compared to neutral pH (pH 6.8 mimicking small intestine conditions), which was partly related to the higher solubility of Fentanyl at low pH [5]. Apart from an increased drug solubility at low pH, geopolymer degradation was also believed to be a contributing cause of the increased Fentanyl release rate [5], a problem that is also present for other clay-based delivery vehicles [11]. Geopolymers are depolymerized [7] in acidic media in a process where the charge balancing cations (Na þ ) in the geopolymer framework are